Streams of flue gas and solid particles entrained therewith are generally discharged from the furnace of a large CFB boiler through multiple flue gas discharge channels to particle separators, usually, cyclone separators, arranged in parallel. Particles separated from the flue gas in the particle separators are returned back to the furnace, while cleaned flue gas is discharged from the particle separators through vertical gas outlet tubes and conducted via a horizontally extending cross over duct system to a back pass. Thermal energy is recovered from the flue gas in the back pass, and cooled flue gas is led from the back pass to different gas cleaning steps and, finally, to a stack, or, in oxyfuel combustion, to carbon dioxide sequestration.
CFB boilers usually have a furnace with a rectangular cross section, in which the width of the long sidewalls is clearly greater than the width of the short sidewalls. In small and medium size CFB boilers, typically having a capacity of about 300 MWe or less, there are usually from one to three particle separators, which are all arranged on one long sidewall of the boiler, and a back pass arranged in line with or opposite to the particle separators. Large size CFB boilers, having a capacity of more than about 300 MWe, usually have multiple particle separators arranged in a row on each of the two opposite long sidewalls of the furnace and a back pass arranged adjacent to a short sidewall of the furnace.
The cross over duct system from the outlet tubes of the particle separators to the back pass of large CFB boilers is usually fairly long, for example, more than thirty meters in the largest CFB boilers of today, and of a heavy construction. Therefore, the cross over duct system has to be well supported, in order to obtain sufficient stability and durability of the construction. Large CFB boilers are usually top-supported, i.e., the furnace, particle separators, and back pass, as well as the cross over duct system, are hanging from a supporting structure surrounding the boiler.
In a CFB boiler having particle separators on both long sidewalls of the furnace, the flue gas outlet tubes of the particle separators arranged on the same sidewall of the furnace are usually connected to a common cross over duct, which conducts the clean flue gases to the back pass. Naturally, such a boiler usually comprises two separate, symmetrically arranged cross over ducts, one on each of the long sidewalls of the furnace. Each of the cross over ducts comprises, conventionally, a main flue gas collecting duct arranged parallel to the long dimension of the horizontal cross section of the furnace and a gas flow bending end portion, for directing the flue gas to an opening in one or more of the sidewalls of the back pass.
Each of the main flue gas collecting ducts of a conventional large circulating fluidized bed boiler collects flue gas from, for example, three or four separators. Thus, the gas flow becomes, especially, at the final sections of a flue gas collecting duct, very high, and potentially eroding, unless the diameter or height of the flue gas duct increases towards the end. Such gradually widening flue gas ducts are, however, usually relatively complicated constructions. Another possibility is that the main flue gas collecting ducts have a constant cross section that is wide enough to maintain a sufficiently low flow velocity even at the end. Such a construction increases the weight of the main flue gas collecting ducts and may cause problems due to the non-constant velocity of the flue gas flow.
The main flue gas collecting ducts are usually arranged outside of the foot-print area of the furnace, especially, above the particle separators. Such cross over ducts then comprise a separate end portion for turning the flue gas streams to the back pass via openings in one or more of the sidewalls of the back pass. The article “Milestones for CFB and OTU Technology—The 460 MWe Lagisza Design Supercritical Boiler Project Update”, presented in CoalGen Conference in Milwaukee, Wis., in August 2007, shows an example of a CFB boiler with a cross over duct system comprising a back pass on a short sidewall of the furnace, flue gas collecting ducts with a constant cross section parallel to the long sidewalls of the furnace and curved end portions for leading the flue gas to openings in the back pass wall facing the short sidewall of the furnace.
U.S. Pat. No. 7,244,400 discloses an alternative solution comprising curved outlet tubes of the particle separators connected to two flue gas collecting channels parallel to the long sidewalls of the furnace arranged at the roof of the furnace. The flue gas collection channels are integrated to the furnace by being constructed utilizing extensions of the furnace walls.
The article “Recent Alstom Power Large CFB and Scale up aspects including steps to Supercritical”, presented at the 47th International Energy Agency Workshop on Large Scale CFB, Zlotnicki, Poland on Oct. 13, 2003, shows a large CFB boiler having three particle separators on each of the long sidewalls, in which the outlet ducts of the particle separators on each side are connected to a complicated cross over duct system comprising a straight collecting channel above the particle separators and a bent flue gas duct portion connecting the center of the collecting channel to the back pass.